October 8th update: Clarification on not currently being able to go back to the stock ATT ROM.

*I take no responsibility if anything happens to your device. That said, me and many other people have done this without issue, so proceed with caution.*
This is a guide to flash GDR2/Amber on your Lumia 920 (RM-820 or RM-821).

Why should I flash my phone?Flashing this ROM has many pros, like:
-No carrier logo on bootup
- there's a way to get FREE internet sharing on at&t if you have any data plan.
-No carrier crapware
-Data Sense, which isn't expected to come to the Lumia 920 on at&t and other carriers
-Ability to install new nokia apps, like Pro cam, Smart cam, Video upload, and Video trimmer.
-No need to wait until your carrier delivers the OTA update
-This ROM gives you FM radio
-You'll receive future OTA updates.

Why shouldn't I flash my phone?-As with all flashing there is a small chance of bricking your device.-Currently, there is no way to go back to official ATT ROM. This may change once/if ATT releases the GDR2 update.-Voids your warrantee if you don't flash back to stock before sending in your device.
-At&t and other carriers *might* deliver this update soon.
-You won't be able to restore a full backup (at least not on AT&T) if you flash this ROM due to the need to switch to LTE 2 through access point, which, like Wi-Fi (FIX THIS MICROSOFT!), doesn't pop up until after you're asked to sign in with your Live ID.
-Some people have reported not being able to use tethering after flashing, even with a tethering plan.Accepting the risks, how do I flash this ROM?

-Don't plug your phone into your computer until I tell you to-
This guide details the process for a 64-bit Windows 8 computer. For other versions of windows, additional steps might be necessary.

Step 6: When it tells you to retry because of lost connection, turn off your phone, Plug it in, and hold down volume down key. Click retry when you hear the short vibration. If asks you if you're sure, and that the product code doesn't match, click Continue.

Step 7: Wait until the ROM fully flashes. DO NOT LOGIN TO MICROSOFT LIVEID AND RESTORE BACKUP ON THIS ROM, then connect to Wifi and unplug the phone, and download the newest OTA update under "phone update" in the phone's settings menu.

Step 8: Wait for the OTA update to download and install. It will take a looooooooooooooooooooooooooong time.

Step 9: In Support Tool, select open product, this time RM-821. Then Programming, Recovery, and select the French Amber ROM.

Step 10: Select Start. When it tells you to retry because of lost connection, Turn off your phone, Plug it in, and hold down volume down key. Press retry.

Step 11: Wait for the flashing to finish. When it boots up, select English (US) to change from French to English.

Step 12: For free tethering on att, go to access point in settings and select "LTE 2". Then download the Spanish (Spain) keyboard and install. After your phone reboots and updates, you should have free tethering. Changing any setting in access point WILL disable tethering until you factory reset your phone. Enjoy GDR2/Amber with an unbranded ROM on your at&t Lumia 920!

Extra Step 2: Many people have had issues with speech-related features after flashing. This includes voice search, speech commands, and voice-to-text texting. In order to get these features back, you need to hold down the windows key to open voice commands, then download the required language pack for speech.

Frequently Asked Questions
1) HELP! I CAN'T GO ONLINE USING CELLULAR!
See step 11.
2) Will I be able to connect to AT&Ts 4G LTE network with this?
See step 11. Yes.
3) Will this void my warranty?
If you don't flash it back to at&t ROM before sending it in, yes.
4) Does Data Sense/FM radio work on at&t on this ROM?
Yes, it does. The official GDR2 for L920 from at&t probably won't include data sense, as L1020 GDR2 doesn't have it.
5) Can I still install at&t specific apps, like myAt&t or at&t locker?
Yes. all AT&T carrier apps still show in the store as "AT&T featured". Cartier apps on WP depend on the sim card that the phone is using to connect to the network, not the firmware. I can download at&t Locker and I've already downloaded myAT&T.
6) Will this SIM re-lock my device or unlock it if its locked?
No. This flashing doesn't SIM unlock the device, nor does it Re-lock it if its already SIM unlocked.
7) Is it possible to revert back to the at&t ROM later?
In theory, after flashing the rogers ROM, you can flash back to any ROM for the Lumia 920. I have not personally tested this, and no one has done it with success yet.

To increase battery life, turn off tap + send in settings (is on by default) and turn on battery saver in settings. Select only when battery is low.Awesome people who helped me make this guide (you should thank/like some of their posts ):-Thanks for providing feedback on your experience and helping other people get updated (and helping me get Windows 8.1 RTM on my Surface Pro and Asus VivoTab RT a bit early ) http://forums.windowscentral.com/mem...gewanderte.htm

I can also confirm that Data Sense is working. If you don't mind loosing your app data, this is a better rom than AT&T will ever provide. Not only is it completely unbranded, but it includes data sense (L1020 with GDR2 on At&t doesn't even have it) and all the great Amber features.

Make sure that you have WinRAR installed, that you extract the folder, and that you rename it "rm 820".
Download link: rm 820 rogers - Download - 4shared - v s
Note that you have to create a free account to download.

Well, Datasense is something ATT will never give you, so there's that.
You let the Amber update now instead of months later, with God knows what things ripped out.
You get a phone without ATT trash apps installed.
No ATT branding.

In all, I'd say it's a cleaner and probably better experience.

Edit: For me it's a no-brainer, ATT replaced my 920 this week so there's nothing on it anyway.